GHSA-6rw7-vpxm-498p qs's arrayLimit bypass in its bracket notation allows DoS via memory exhaustion

• npm qs: < 6.14.1 → fixed in 6.14.1

Summary The `arrayLimit` option in qs did not enforce limits for bracket notation (`a[]=1&a[]=2`), only for indexed notation (`a[0]=1`). This is a consistency bug; `arrayLimit` should apply uniformly across all array notations. Note: The default `parameterLimit` of 1000 effectively mitigates the DoS scenario originally described. With default options, bracket notation cannot produce arrays larger than `parameterLimit` regardless of `arrayLimit`, because each `a[]=value` consumes one parameter slot. The severity has been reduced accordingly. Details The `arrayLimit` option only checked limits for indexed notation (`a[0]=1&a[1]=2`) but did not enforce it for bracket notation (`a[]=1&a[]=2`). Vulnerable code (`lib/parse.js:159-162`): ```javascript if (root === '[]' && options.parseArrays) { obj = utils.combine([], leaf); // No arrayLimit check } ``` Working code (`lib/parse.js:175`): ```javascript else if (index <= options.arrayLimit) { // Limit checked here obj = []; obj[index] = leaf; } ``` The bracket notation handler at line 159 uses `utils.combine([], leaf)` without validating against `options.arrayLimit`, while indexed notation at line 175 checks `index <= options.arrayLimit` before creating arrays. PoC ```javascript const qs = require('qs'); const result = qs.parse('a[]=1&a[]=2&a[]=3&a[]=4&a[]=5&a[]=6', { arrayLimit: 5 }); console.log(result.a.length); // Output: 6 (should be max 5) ``` Note on parameterLimit interaction: The original advisory's "DoS demonstration" claimed a length of 10,000, but `parameterLimit` (default: 1000) caps parsing to 1,000 parameters. With default options, the actual output is 1,000, not 10,000. Impact Consistency bug in `arrayLimit` enforcement. With default `parameterLimit`, the practical DoS risk is negligible since `parameterLimit` already caps the total number of parsed parameters (and thus array elements from bracket notation). The risk increases only when `parameterLimit` is explicitly set to a very high value.